KVM: selftests: Add wrapper macro to handle and assert on expected SIGBUS

Extract the guest_memfd test's SIGBUS handling functionality into a common
TEST_EXPECT_SIGBUS() macro in anticipation of adding more SIGBUS testcases.
Eating a SIGBUS isn't terrible difficult, but it requires a non-trivial
amount of boilerplate code, and using a macro allows selftests to print
out the exact action that failed to generate a SIGBUS without the developer
needing to remember to add a useful error message.

Explicitly mark the SIGBUS handler as "used", as gcc-14 at least likes to
discard the function before linking.

Opportunistically use TEST_FAIL(...) instead of TEST_ASSERT(false, ...),
and fix the write path of the guest_memfd test to use the local "val"
instead of hardcoding the literal value a second time.

Suggested-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Tested-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Lisa Wang <wyihan@google.com>
Tested-by: Lisa Wang <wyihan@google.com>
Link: https://lore.kernel.org/r/20251003232606.4070510-12-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kvm/guest_memfd_test.c

@@ -14,8 +14,6 @@
 #include <linux/bitmap.h>
 #include <linux/falloc.h>
 #include <linux/sizes.h>
-#include <setjmp.h>
-#include <signal.h>
 #include <sys/mman.h>
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -77,30 +75,16 @@ static void test_mmap_supported(int fd, size_t total_size)
 	kvm_munmap(mem, total_size);
 }
 
-static sigjmp_buf jmpbuf;
-void fault_sigbus_handler(int signum)
-{
-	siglongjmp(jmpbuf, 1);
-}
-
 static void test_fault_overflow(int fd, size_t total_size)
 {
-	struct sigaction sa_old, sa_new = {
-		.sa_handler = fault_sigbus_handler,
-	};
 	size_t map_size = total_size * 4;
 	const char val = 0xaa;
 	char *mem;
 	size_t i;
 
 	mem = kvm_mmap(map_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd);
 
-	sigaction(SIGBUS, &sa_new, &sa_old);
-	if (sigsetjmp(jmpbuf, 1) == 0) {
-		memset(mem, 0xaa, map_size);
-		TEST_ASSERT(false, "memset() should have triggered SIGBUS.");
-	}
-	sigaction(SIGBUS, &sa_old, NULL);
+	TEST_EXPECT_SIGBUS(memset(mem, val, map_size));
 
 	for (i = 0; i < total_size; i++)
 		TEST_ASSERT_EQ(READ_ONCE(mem[i]), val);

tools/testing/selftests/kvm/include/test_util.h

@@ -8,6 +8,8 @@
 #ifndef SELFTEST_KVM_TEST_UTIL_H
 #define SELFTEST_KVM_TEST_UTIL_H
 
+#include <setjmp.h>
+#include <signal.h>
 #include <stdlib.h>
 #include <stdarg.h>
 #include <stdbool.h>
@@ -78,6 +80,23 @@ do {									\
 	__builtin_unreachable(); \
 } while (0)
 
+extern sigjmp_buf expect_sigbus_jmpbuf;
+void expect_sigbus_handler(int signum);
+
+#define TEST_EXPECT_SIGBUS(action)						\
+do {										\
+	struct sigaction sa_old, sa_new = {					\
+		.sa_handler = expect_sigbus_handler,				\
+	};									\
+										\
+	sigaction(SIGBUS, &sa_new, &sa_old);					\
+	if (sigsetjmp(expect_sigbus_jmpbuf, 1) == 0) {				\
+		action;								\
+		TEST_FAIL("'%s' should have triggered SIGBUS", #action);	\
+	}									\
+	sigaction(SIGBUS, &sa_old, NULL);					\
+} while (0)
+
 size_t parse_size(const char *size);
 
 int64_t timespec_to_ns(struct timespec ts);

tools/testing/selftests/kvm/lib/test_util.c

@@ -18,6 +18,13 @@
 
 #include "test_util.h"
 
+sigjmp_buf expect_sigbus_jmpbuf;
+
+void __attribute__((used)) expect_sigbus_handler(int signum)
+{
+	siglongjmp(expect_sigbus_jmpbuf, 1);
+}
+
 /*
  * Random number generator that is usable from guest code. This is the
  * Park-Miller LCG using standard constants.